Conventional oil-based ballpoint pen inks are high viscosity inks with an ink viscosity of 6000–15,000 mPa·s, composed mainly of benzyl alcohol and phenyl cellosolve mixed solvents. However, because of their high viscosity, such oil-based ballpoint pen inks have had a heavy writing feel, such that the ink cannot be smoothly transferred to the page surface without high writing pressure. The written ink residing on the page surface also produces poorly dried drawn lines, or the ink wets and adheres to the pen tip and accumulated ink transfers to the page surface, often smudging the hand. This is known as “ink blotting”. As a result of this ink blotting, such inks have not been suitable for use in large-diameter ballpoint pens.
In order to improve the writing feel of ballpoint pens and reduce ink blotting, there have been proposed low viscosity inks with an ink viscosity of 2000–10,000 mPa·s at 20° C. comprising polyvinyl pyrrolidone (molecular weight: 1,000,000 or greater) as the stringy polymer (Japanese Unexamined Patent Publication (Kokai) HEI No. 8-157765 and elsewhere). However, while such low viscosity inks provide a better writing feel, ink blotting still occurs readily and the ink also tends to bleed through PPC paper and the like.
Low viscosity note-writing oil-based ink, used for marking pens, is composed mainly of low aliphatic alcohols and has a viscosity of 3–10 mPa·s. However, such low viscosity ink for marking pens is mainly used for non-adhering surfaces such as plastic, metal or glass, and although it dries quickly, the ink transfer volume is high and the viscosity is low, creating a problem of “color bleed-through” when used for ballpoint pens or on ordinary paper, typically PPC.
Oil-based ballpoint pens are usually loaded with a refill cartridge which has its axial cylinder filled with ink and is fitted in the ball tip at the front end thereof. Most ballpoint pens have the back end exposed to the air.
With refill cartridges thus exposed to the air, the volatile components in the ink evaporate often leading to a change in the composition of the ink, and in the case of inks that have been prepared with a delicate balance of components, it is sometimes impossible to achieve the performance expected for the ink design, particularly with inks comprising low boiling point, highly volatile components.
It is an object of the present invention to provide an oil-based ballpoint pen ink composition which, for oil-based ballpoint pens on both low-absorption surfaces and high-absorption surfaces, exhibits an improved writing feel and accelerated drying of drawn lines, produces low hand smudging, allows writing without bleed-through of the written ink as occurs with conventional felt pens, exhibits excellent ink conserving quality, and can be suitably used even with a large flow volume of ink for note writing, such as in the case of thick-line writing pens, as well as to provide an oil-based ballpoint pen employing the composition.